InAlAsSb/InGaSb and InAlPSb/InGaSb heterojunction bipolar transistors

ABSTRACT

This invention pertains to heterojunction bipolar transistors containing a semiconductor substrate, a buffer layer of an antimony-based material deposited on the substrate, a sub-collector layer of an antimony-based material deposited on the buffer layer, a collector layer of an antimony-based material deposited on the sub-collector layer, a base layer of an antimony-based material deposited on the collector layer, an emitter layer of an antimony-based material deposited on the base layer, and a cap layer of an antimony-based material deposited on the emitter layer.

CROSS REFERENCE TO RELATED APPLICATIONS

This patent application claims the benefit of U.S. ProvisionalApplication No. 60/615,119 filed Sep. 30, 2004.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention pertains to heterojunction bipolar transistors (HBTs)that employ advanced antimony-based material designs.

2. Description of Related Art

Future generations of microwave and millimeter-wave radar,communications, electronic warfare, smart weapons and imaging systemswill require higher precision, smaller size, increased bandwidth, loweroperating voltages and lower cost of production. To meet the demand forimproved high-frequency performance, considerable effort within the pastten years or so has focused on the development of GaAs-based andInP-based heterojunction bipolar transistors. As a result, a variety ofHBT analog and digital circuits have been fabricated which exhibithigher gain and switching speeds with lower power dissipation. Some ofthe primary factors responsible for the improved HBT performance havebeen the use of InGaAs or GaAsSb materials in the base to improve thecharge transport through the base and to lower the emitter-base andcollector-base voltages needed to obtain a high collector current. As aresult of these improvements, InP-based HBTs have distinct high-speedlogic circuit performance advantages compared to GaAs-based HBTs andthey have set records for the maximum frequency of operation for anythree-terminal semiconductor device.

In the longer term, HBTs that employ In_(z)Ga_(1-z)Sb in the base andIn_(x)Al_(1-x)As_(1-x)As_(y)Sb_(1-y) or In_(x)Al_(1-x)P_(y)Sb_(1-y)material in the emitter along with In_(z)Ga_(1-z)Sb, orIn_(v)Al_(1-v)As_(w)Sb_(1-w) or In_(v)Al_(1-v)P_(w)Sb_(1-w) in thecollector may be more attractive than InP-based HBTs for some of theabove applications due to the substantially improved electronicproperties of these new material systems.

Objects and Brief Summary of the Invention

It is an object of this invention to provide transistors that employadvanced material designs using antimony-based materials to increaseoperating speeds.

It is another object of this invention to provide heterojunction bipolartransistors using antimony-based materials to reduce power dissipation.

It is another object of this invention to provide heterojunction bipolartransistors wherein base, emitter and collector layers in each is anantimony-based material.

It is another object of this invention to provide heterojunction bipolartransistors that can operate at switching speed on the order of 500 GHzand are suitable for logic circuits.

These and other objects of this invention can be attained byheterojunction bipolar transistors that use antimony-based materials inthe collector, the base and the emitter layers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional schematic representation of a layer diagramof a heterojunction transistor of this invention.

FIG. 2 is a band diagram of the antimony-based heterojunction bipolartransistors of this invention.

DETAILED DESCRIPTION OF THIS INVENTION

Heterojunction bipolar transistors of this invention employ advancedmaterial layer designs to increase operating speed and reduce powerdissipation. The HBTs utilize InAlAsSb or InAlPSb for the emitterlayers, and InGaSb for the base layers and InAlAsSb, InAlPSb and InGaSbfor the collector layers. As a result, the transistor devices of thisinvention exhibit high frequency performance and operate at lower biasvoltage. More specifically, the HBTs of this invention, where electronsflow vertically through the HBT device as shown in FIG. 1, include, asshown in FIG. 1, a semi-insulating substrate 12 of GaAs, or InP of theHBT device 10; a buffer layer 14 of un-doped AlSb deposited on thesubstrate; sub-collector layer 16 of highly doped (n⁺) InAlAsSb,InAlPSb, InGaSb, or InAsSb deposited on the buffer layer; collectorlayer 18 of doped (n) InAlAsSb, InAlPSb, or InGaSb deposited on thesub-collector layer; base layer 20 of highly doped (p⁺) InGaSb; andemitter layer 22 of doped (n) InAlAsSb or InAlPSb deposited on the baselayer; and cap layer 24 of highly doped (n⁺) InGaSb, InAsSb, or InAsdeposited on the emitter layer. In a preferred embodiment, the molfraction x in the material In_(x)Ga_(1-x)Sb are in the range of x of 0.1to 0.5. In materials In_(x)Al_(1-x)As_(y)Sb_(1-y) andIn_(x)Al_(1-x)P_(y)Sb_(1-y), mol fractions x and y are in the ranges of0.4 to 0.8 for x and 0.2 to 0.5 for y. Thickness of the various layersis typically in the following ranges:

-   -   substrate 12, 475-525 μm    -   buffer 14, 1-2 μm    -   sub-collector 16, 2000-8000 Å    -   collector 18, 2000-10000 Å    -   base 20, 200-1000 Å    -   emitter 22, 1000-4000 Å    -   cap 24, 100-500 Å

Function of the substrate is obviously to provide support for the layersdeposited thereon. Available substrates include GaAs, InP and GaSb.Lattice constants of the first two are in the range of 5.7-5.9 Å andthey are semi-insulating (SI) whereas lattice constant of GaSb is 6.1,which is closest to the desired lattice constant and that is why GaSbwhich is unintentionally doped p-type is sometimes used in the proof ofthe principle experiments.

Function of the buffer layer is to accommodate lattice mismatch betweenlayers of the device and the substrate. The lattice mismatch function isthe principal reason why the buffer layer thickness is typically on theorder of 1 to 2 μm.

In reference to the sub-collector layer, a high conductivity layer isneeded to take current out of the device. This is facilitated by highlydoping the sub-collector layer with tellurium and choosing asemiconductor with a high electron mobility. The high doping level, andthe choice of a narrow bandgap semiconductor help to produce goodquality ohmic metal contacts to the collector. While the main functionof the sub-collector layer is to carry current out of the device, it canalso be used, i.e., double use, as a portion of the buffer layer toaccommodate lattice mismatch.

In reference to the collector layer, one needs to set the conductionband alignment between the base layer and the collector layer to getgood device performance. The bandgap of the collector needs to be largeenough to support the voltage drop across the collector. The dopinglevel, electron mobility and layer thickness should be appropriate sothat electrons can be transported there quickly across the collector.Function of the collector layer is to collect electrons that travelthrough the base layer.

By changing potential applied to the highly doped (p⁺) base layer, onegets current to flow from the emitter layer to the collector layer.Typically the InGaSb base is heavily doped with Be to about 10¹⁹ cm⁻³and it is on the order of 200 to 700 Å thick.

The base layer is necessary for operation of this device since thebase-emitter forms a forward based p-n junction and the base-collectorforms a reverse biased p-n junction. When a bias is applied to the baseit controls the current that flows from the emitter to the collector.

The emitter layer has to have its conduction and valence bands carefullytailored to the conduction and valence bands in the base to optimize theHBT's performance. A wide range of conduction band offsets relative tothe base is available with the quaternary material (InAlAsSb) byadjusting the In/Al and As/Sb ratios while maintaining the lattice matchof the emitter and base. In addition, a desirable large valence bandoffset is maintained, almost independent of the In/Al and In/Alcomposition. The emitter doping is important in order to obtain a largecurrent flow to the collector. Near the base the emitter doping helps todetermine the depletion layer thickness. The doping level also helps tominimize the resistance of the emitter. The emitter layer needs to bethin and have a high conductivity to limit parasitic series resistances.Similarly, it is important to have low resistance ohmic contacts to theemitter. The cap layer is used to help make low resistance ohmiccontacts to the emitter layer. The cap needs to be composed of a heavilydoped narrow bandgap semiconductor in order to have quality ohmiccontacts between the external metal wires and the emitter layer

Advantages of this invention include attractive material and designfeatures, such as: InGaSb band-gap that can be tailored from 0.3 to 0.7eV for optimum low voltage operation at emitter-base voltages from 0.1to 0.5 V; the high hole mobility in the InGaSb results in low parasiticbase resistance; a large valence band offset from 0.3 to 0.45 eV at theemitter prevents unwanted hole currents from flowing from the base tothe emitter; the InAlAsSb composition of the emitter can be chosen tooptimize the conduction band offset to the base; the chemical etchingdifferences between InGaSb and InAlAsSb can be used to optimize an etchstop fabrication process; the InGaSb Fermi-level is pinned near thevalence band edge at the surface and this will help in forming lowresistance non-alloyed ohmic contacts to the InGaSb.

In reference to fabrication of the HBT devices, the HBT materials can begrown metamorphically on a GaSb or semi-insulating, GaAs or Inpsubstrate by molecular beam epitaxy. They may also be grown by otherepitaxial methods such as organo metallic vapor deposition. After thegrowth of a thick AlSb buffer layer, a heavily doped n+ InAlAsSb,InAlPSb, InAsSb or InGaSb subcollector is grown, followed by amoderately doped n-type InAlAsSb, InAlPSb or InGaSb collector, then anp+ InGaSb base, followed by an InAlAsSb or InAlPSb emitter. Finally, ann+ InGaSb, InAs or InAsSb cap layer is grown above the emitter layer.The bandgaps and band alignments for these material combinations wereobtained using data from 8-band k-p simulations, as illustrated in FIG.2.

For a basic disclosure on the general fabrication of a device of thisnature, see Boos et al U.S. Pat. No. 5,798,540 (??), the entire contentsof which is incorporated herein.

InGaSb HBTs have some very attractive advantages compared toconventional GaAs or InP-based HBTs. The key features of this new designare the use of a narrow bandgap InGaSb base layer with either materialfor the emitter, and InAlAsSb, InAlPSb or InGaSb for the collector. TheInGaSb base has a higher hole mobility and electron mobility than InGaAsor GaAsSb which are used for the base in HBTs lattice matched to InP.The higher hole mobility will result in lower resistance between theohmic contact to the base layer and the emitter. Minimizing thisresistance is important in obtaining high frequency operation. The highelectron mobility in the base is also important as it leads to fastelectron transit across the base from the emitter to the collector. Thisis also important for high frequency operation.

Another key advantage of this approach is the design flexibility gainedby being able to optimize the conduction band offsets at theInAlAsSb/InGaSb or InAlPSb/InGaSb heterojunctions while maintaining alarge valence band offset at one lattice constant for the complete HBT.An attractive feature of the InGaSb/InAlAsSb is that for a particularlattice constant, the valence band offset does not change very much withas the quaternary composition changes through its entire possible range,Over the composition range the quaternary band gap varies from 0.15 toabout 1.5 eV. Because of the constant valence band offset the bandgapvariation results in a large variation in the conduction band offset.Tuning the conduction band offsets at the emitter-base andcollector-base contacts is important in optimizing device operation.This design flexibility results in higher emitter efficiency, highercurrent gain and improved electron transport through the transistor. Thepreliminary band diagrams that show how the composition of the InGaSbHBTs change when moving from a lattice constant of 6.13A to 6.3A, areshown in FIG. 2 to illustrate the large lattice constant window foroptimization of this HBT.

In addition to the increased high-speed performance, InGaSb HBTs arealso attractive for applications requiring low-collector voltageoperation. The lower bandgap of InGaSb reduces the emitter-base voltagerequired to reach a given collector current density compared to InP andGaAs based HBTs. This leads to lower power dissipation than those HBTs.

The material growth and fabrication technology for InAlAsSb/InGaSb HBTshas been demonstrated. Better determination of the appropriatecompositions and doping levels for the different layer structures arerequired to optimize performance. Leakage currents associated withgrowth defects near the emitter-base and base-collector junctions willneed to be reduced. When using narrow bandgap collector material impactionization in the collector will also need to be addressed.

HBT improved performance with these improved material design featureswas unexpected compared to InP-based and GaAs based HBTs. By being ableto vary the alloy composition and thickness of the layer structure, awide range of possibilities exist to exploit the use of these materialsin the device. The unique combinations of these heterojunction materialsmake InGaSb HBTs attractive candidates in future technologies where highspeed, gain, and efficiency at low bias voltage will be required.

While presently preferred embodiments have been shown for these novelheterojunction bipolar transistors, and of the several modificationsdiscussed, persons skilled in this art will readily appreciate thatvarious additional changes and modifications can be made withoutdeparting from the spirit of the invention, as defined anddifferentiated by the following claims.

1. An electronic device comprising a semiconductor substrate, a bufferlayer deposited on said substrate, a sub-collector layer deposited onsaid buffer layer, a collector layer of an antimony-based materialdeposited on said sub-collector layer, a base layer of an antimony-basedmaterial deposited on said collector layer, an emitter layer ofantimony-based material deposited on said base layer, and a cap layerdeposited on said emitter layer.
 2. The device of claim 1 wherein saidbuffer layer is un-doped.
 3. The device of claim 2 wherein said bufferlayer is doped.
 4. The device of claim 3 wherein said substrate isselected from the group consisting of GaSb, GaAs and InP.
 5. The deviceof claim 4 wherein said cap layer is highly doped to the extent of onthe order 1×10¹⁹ eV/cm³ tellurium or silicon.
 6. The device of claim 4wherein said buffer layer material is AlSb, step or continuously gradedInAlSb, or InAlAsSb or containing portions that are short periodsuperlattices; said collector layer is selected from the groupconsisting of InAlAsSb, InAlPSb and InGaSb; and said base layer isInGaSb.
 7. The device of claim 6 wherein said sub-collector and saidcollector layers are selected from the group consisting of InAlAsSb,InAlPSb, InAsSb and InGaSb and said cap layer is selected from the groupconsisting of InGaSb, InAs and InAsSb.
 8. The device of claim 7 whereinsaid cap layer is highly doped with tellurium or silicon to the level ofon the order of 1×10¹⁹ eV/cm³.
 9. The device of claim 8 wherein saidbuffer layer includes un-doped layer of InAlAsSb, InAlSb, or step orcontinuously graded InAlAsSb, InAlSb, with portions containing shortperiod superlattice structures.
 10. The device of claim 9 that can havefrequency on the order of 500 GHz and a reduced power dissipation,wherein thickness of the various components is as follows: substrate,475 to 525 μm; buffer layer, 1 to 3 μm; sub-collector layer, 2000-8000Å; collector layer, 2000-8000 Å, base layer, 200-800 Å emitter layer,1000-4000 Å, and cap layer, 100-500 Å.
 11. Heterojunction bipolartransistor containing a semiconductor substrate, a buffer layerdeposited on said substrate, a sub-collector layer deposited on saidbuffer layer, a collector layer of an antimony-based material depositedon said sub-collector layer, a base layer of an antimony-based materialdeposited on said collector layer, an emitter layer of an antimony-basedmaterial deposited on said base layer, and a cap layer deposited on saidemitter layer.
 12. The transistor of claim 11 wherein said buffer layeris un-doped.
 13. The transistor of claim 12 wherein said buffer layer isAlSb, InAlAsSb, InAlSb, or step or continuously graded InAlAsSb, InAlSb,with portions containing short period superlattice structures.
 14. Thetransistor of claim 12 wherein said substrate is selected fro the groupconsisting of GaSb, GaAs and InP.
 15. The transistor of claim 14 whereinsaid cap layer is highly doped to the extent of on the order 1×10¹⁹eV/cm³ tellurium or silicon.
 16. The transistor of claim 14 wherein saidsub-collector and said collector layers are selected from the groupconsisting of InAlAsSb, InAlPSb and InGaSb; said base layer is InGaSb;and said emitter layer is selected from the group consisting of InAlAsSband InAlPSb materials.
 17. The transistor of claim 16 wherein saidsub-collector layer is highly doped n⁺ material, said collector layer isdoped n material and said base layer is highly doped p⁺ material. 18.The transistor of claim 17 wherein said cap layer is highly doped withtellurium or silicon to the level of on the order of 1×10¹⁹ eV/cm³. 19.The transistor of claim 18 wherein said buffer layer includes un-dopedlayer of AlSb, InAlAsSb, InAlSb, or step or continuously gradedInAlAsSb, InAlSb, with portions containing short period superlatticestructures.
 20. The transistor of claim 19 that can have frequency onthe order of 500 GHz and a reduced power dissipation, wherein thicknessof the various components is as follows: substrate, 475 to 525 μm;buffer layer, 1 to 3 μm; sub-collector layer, 2000-8000 Å collectorlayer, 2000-8000 Å base layer, 200-800 Å emitter layer, 1000-4000 Å, andcap layer, 100-500 Å.